The present invention relates to polymer membranes for separation processes, and more particularly to extrusion methods for making semi-permeable polymer membranes for separation processes from high temperature polymers.
The various filtration and/or separation processes utilizing porous membranes include concentration-driven separations such as dialysis, electromembrane separations, used to separate dissolved charged ions, and pressure-driven separations, including the familiar processes of micro-filtration, ultra-filtration, and hyperfiltration. Pressure-driven processes achieve the separation of suspended or dissolved particles of different sizes as the result of the capability of the particles to penetrate through or be retained by semi-permeable porous membranes of varying permeabilities and porosities. The porosity of the membrane determines whether the separation is termed micro-filtration, ultra-filtration or hyper-filtration.
Polymer membranes are used on a large scale in many industrial processes, including the desalination of sea water, the cleaning of industrial effluents, the fractionation of macro-molecular solutions in the food and drug industries, and the controlled release of drugs in medicine. Micro-filtration membranes are used for the filtration of particles in the 0.1-2 micron size range, whereas ultra-filtration membranes can trap particles in the 0.001-0.1 micron size range.
Composite membranes are also known, a common example being asymmetric ultrafiltration membranes comprising a thin particle-selective skin or surface membrane of fine porosity disposed for physical support on a backing plate of coarser porosity. Such composites improve mass transport in processes such as ultra-filtration and reverse osmosis, wherein very fine pore sizes must be provided.
Conventional methods for making microporous membranes include the sintered particle method, wherein powders of a selected membrane material are sintered to provide porous films or plates, typically with thicknesses in the 100-500 micrometer range. Ceramics, glasses, metals, and polymers such as polytetrafluoroethylene, can be formed into microporous membranes by this technique, although it generally yields structures of relatively low porosity, e.g., in the range of 10-40% by volume.
Alternative methods for making microporous polymer membranes include the stretching of homogeneous polymer films to cause partial fracture and the formation of a fine pore structure therein, and irradiation methods wherein films are irradiated with charged atomic particles and then etched to enlarge the particle tracks therein to pores. Also used is a phase inversion process wherein an immiscible liquid is introduced into a thin liquid film of a polymer solution, and the polymer then precipitated as a polymer film comprising a network of pores resulting from the presence of the immiscible phase during precipitation.
The above-described chemical methods for membrane fabrication generally use polymers that can be etched or otherwise dissolved in controlled fashion, and therefor are inherently limited to polymers which have some solubility in organic solvents. This limits the durability of the membranes which can be provided.
In a relatively recent development, a method for providing semi-permeable membranes from high strength, high temperature polymers such as polyether ketones, poly(aryl ether) ketones, and liquid crystal polymers has been developed. That method involves melt-blending a batch mixture containing a finely divided high-temperature polymer and a finely divided leachable glass to form a two-phase glass-polymer blend comprising continuous glass and continuous polymer phases.
The blend thus provided is then formed into a glass-polymer body of a selected shape such as a plate or membrane, and then treated with an aqueous leaching medium to leach the continuous intercommunicating glass phase from the continuous polymer phase. The product is a microporous polymer body comprising an intercommunicating pore structure consisting of a relict polymer network, generally having a pore volume in excess of 50% and average pore diameters in the range of about 0.1-10 micrometers. Most typical porosities are in the range of 70 to 80% by volume, with mean pore sizes of 0.1 to 0.7 microns and a narrow pore size distribution.
The method described above is disclosed and claimed in a copending, commonly assigned patent application of G. H. Beall et. al., Ser. No. 808,814 filed Dec. 17, 1991 for "Polymer Membranes for Separation Processes", now U.S. Pat. No. 5,183,607, and that patent is expressly incorporated herein by reference for a further description of such products and methods.
The method of the above-described patent application provides membranes of excellent quality, particularly when the membranes are produced by pressing of the glass-polymer blend followed by leaching of the resulting sheets or films. However, it has recently been found that membranes produced by extrusion of the glass-polymer blends followed by leaching exhibit significantly lower permeability than membranes of same composition produced from pressed films. Hence, although glass removal from the films appears to be substantially complete, with polymer residues and leached film densities essentially equivalent to those obtained in the case of pressed films, the extruded products still differ substantially from the pressed products in terms of permeability.
This represents a significant problem because extrusion is a preferred method for the production of melt-blended glass/polymer film precursors, particularly for applications requiring large quantities of semi-permeable membrane materials. Hence, extrusion can most conveniently provide large quantities of film material at low cost, in uniform thickness, and with relatively high film quality. Accordingly, a method for extruding blended thin-film material films which would provide products with properties equivalent to those of pressed plates or films is needed.
It is therefore a principal object of the present invention to provide an improved method for the production of semi-permeable microporous membranes utilizing extrusion processing.
It is a further object of the invention to provide an extrusion method which provides semi-permeable microporous membranes of controllable permeability on a repeatable basis.
Other objects and advantages of the invention will become apparent from the following description thereof.